Blue LEDs using nitride semiconductors have already been provided for practical use. Recently, it becomes possible to provide a practical blue laser diode made of nitride semiconductor by using a GaN substrate.
The inventors have disclosed a nitride semiconductor laser diode using a GaN substrate in, for example, Japanese Journal of Applied Physics. Vol.37(1998) pp.L309–L312. The GaN substrate can be formed, for example, by the following method: A GaN layer is formed on a sapphire substrate and a protective film made of SiO2 is formed partially on the surface of the GaN film. Then, GaN is grown again on the GaN film and the sapphire substrate is removed. The secondly-formed GaN layer grows mainly in a lateral direction, so that a proceeding of dislocations is prevented. By using this method, a GaN substrate having low dislocation density can be obtained. The nitride semiconductor laser device made with such a low dislocation-density GaN substrate showed continuous-wave oscillation and can be operated continuously for more than ten thousand hours.